1. Field of the Invention
An aspect of the present invention relates to a method of wiring data transmission lines, and more particularly to a method of wiring data transmission lines between a CPU and an SDRAM.
2. Description of the Related Art
Printed circuit boards are substrates on which chips or other electronic components are mounted. Printed circuit boards are made of reinforced fiberglass or plastic, and have a copper circuit that connects the components to each other. The printed circuit board having the most important role in a system is typically referred to as a main board or a motherboard, and other printed circuit boards having lesser roles that are inserted into slots of the main board are typically referred to as boards or cards. Although early printed circuit boards were designed for mounting individual components thereon, recent printed circuit boards have been designed for mounting very large-scale integration (VLSI) integrated circuits thereon.
When producing a printed circuit board, a thin copper film adhered to the reinforced fiberglass or plastic substrate is coated with a photoresist. The photoresist is illuminated with light through a film having a circuit pattern thereon to form a latent image of the circuit pattern on the photoresist. The latent image is developed, and the portions of the photoresist not forming part of the circuit pattern are dissolved and washed away. The substrate is immersed into an etching bath, and portions of the copper film not protected by the remaining photoresist are etched away, thereby forming the same circuit pattern on the copper film as the circuit pattern on the film.
The main board of a computer has a connector that is used to connect peripheral modules to a bus. In general, the main board has integrated circuits such as a CPU, memory devices, a system controller, and the like mounted thereon. The CPU, the memory devices, the system controller, and the like perform various data processing operations to perform commands. To enable this, data transmission lines are connected between the integrated circuits to transmit and receive large amounts of data.
Representative examples of data transmission lines include data transmission lines between the CPU and the memory devices. For serial data transmission, data can be transmitted through only one or two data transmission lines, whereas for parallel data transmission, data is generally transmitted on a byte-by-byte basis (where, for example, one byte includes eight bits) through a number of data transmission lines corresponding to the number of bits in one byte.
Because of data mapping and consistency in wiring a number of data transmission lines between the CPU and the memory devices, wiring is frequently performed to match pin numbers of data input and output terminals of the CPU to pin numbers of data input and output terminals of the memory devices.
However, when wiring the data transmission lines after matching the pin numbers, crossing of the data transmission lines inevitably occurs due to the presence of other components mounted around the CPU and the memory devices. As a result of this, the data transmission lines are bypassed to a rear side of the substrate through via holes to prevent short circuit of the data transmission lines caused by the crossing of the data transmission lines.
However, the via holes cause harmonic waves, i.e., noise, and thus negatively influence reliability of data transmission. In addition, bypassing of the data transmission lines through the via holes causes an increase in length of some unit-bit data transmission lines (i.e., data transmission lines corresponding to individual bits of a data transmission unit of one byte), resulting in non-uniform data transmission of the bits within one byte. In addition, when stacking a plurality of substrates, an impedance difference between the substrates can cause other problems.